High frequency vacuum tube with closely spaced cathode and non-emissive grid

Abstract: A vacuum tube for handling an r.f. signal having a predetermined frequency range comprises a linear electron beam emitting cathode (136), a heater (166) and a non-electron emissive current modulating grid (138). The grid (138) is positioned from the cathode (136) by the distance an emitted electron from the cathode (136) can travel in a quarter cycle of the r.f. signal. Outer (202) and inner (204) coaxial metal tubes forming a resonant line of a signal coupler (143) are respectively connected to the grid (138) and cathode (136) so electrons passing through the grid (138) are in bundles in an interaction region (20) between an accelerating anode (18) and the grid (138). Ferrite tiles (42) absorb r.f. fields in the interaction region (20). In one embodiment a signal coupling loop (12) is between metal tubes (202,204) at an end of the tubes spaced 3λ/4 from the grid (138) and cathode (136). In a second embodiment the coupler (143) includes a low voltage coaxial line (110) having an inner conductor (112) connected to a first metal face (125), spaced from a second opposed metal (134) face by a solid dielectric (140). The coaxial outer conductor (114) is connected to a third metal face (128), spaced from a fourth opposed metal face (142) by the solid dielectric (140). The third (128) and fourth (142) faces surround the first (125) and second (134) faces. The first (115) and third (128) faces are at DC ground potential while the second (134) and fourth (142) faces are at high negative DC voltages. The second (134) and fourth (142) faces are respectively at common ends of interior (204) and exterior (202) coaxial metal tubes forming a λ/2 coupler (143). Other ends of the tubes (202,204) are connected to the cathode (136) and grid (138). Bias leads (190,194) for the grid (138) and cathode (136) are connected to the exterior (202) and interior (204) tubes at positions λ/4 from the grid (138) and cathode (136), while a heater lead (196) goes through the interior tube (204) at a position λ/4 from the grid (138) and cathode (136).

Description: The present invention relates generally to high frequency vacuum tubes including a cathode closely spaced to a non-emissive grid coupled via a resonant structure to an r.f. signal to be amplified and more particularly to such a tube with at least one of (1) an r.f. field absorbing material substantially surrounding an interaction region between the grid and an accelerating anode, (2) a loop between a pair of coaxial resonant tubes coupling the signal to the grid and cathode, (3) capacitive coupling to a pair of coaxial resonant tubes coupling the signal to the grid and cathode, or (4) bias leads for the grid and cathode respectively connected to outer and inner resonant coaxial metal r.f. coupling tubes at a point n 1 λ/4 from the grid and cathode, in combination with a heater lead extending through the inner tube at a point n 1 λ/4 from the grid and cathode, where n 1 is an odd integer and λ is the wavelength of the r.f. signal supplied to the grid and cathode by the inner and outer tubes. A recently developed vacuum tube for handling r.f. signals includes a cathode for emitting a linear electron beam, a grid positioned parallel and in close proximity to the cathode (no farther than the distance an emitted electron can reach in a quarter cycle of a signal being handled by the tube) for current modulating the beam, and a cavity resonant to the frequency of the signal positioned between the grid and a collector electrode for the beam. The grid is coupled by a structure resonant to the frequency being handled by the tube to an input of the tube. Very high efficiency is achieved with such a tube by biasing the grid so current flowing from the cathode toward the grid occurs for no more than one half cycle of the r.f. signal handled by the tube. The grid is formed of a non-electron emissive material, such as pyrolytic graphite or molybdenum. In one prior art configuration, a resonant input circuit supplies electric fields in opposing phase between the cathode and grid and between the grid and an accelerating anode positioned between the grid and an output cavity. In another prior art device, a second resonant cavity positioned between the output cavity and the accelerating anode is adjusted so the resonant frequency thereof is above the frequency being handled by the tube, to increase the average efficiency of the tube. These prior art structures are disclosed in the commonly assigned US-A-4,480,210, 4,527,091 and 4,611,149. Commonly assigned patent applications generally dealing with similar tubes are Lien et al., US-A-5,317,233, Serial No. 07/508,442, filed April 13, 1990 and Lien, US-A-5,233,269, Serial No. 07/508,611, also filed April 13, 1990. Commercially available tubes of this type have included a resonant structure for coupling the input signal to the cathode-grid assembly in the form of a resonant cavity coaxial with the cathode and the electron beam emitted from it. This resonant cavity has a length in the direction of the beam axis that is nominally either a half or full wavelength at the frequency handled by the tube. In practice, it is most usually at the full wavelength handled by the tube, causing the tube to have a relatively long length. The input signal to the cavity is capacitive-coupled to the cavity. A metal structure in the input resonant cavity couples the field established in the cavity in response to the input signal to the grid. An r.f. electric field is thereby established between the grid and cathode, to current-modulate the electron beam. An r.f. field is also established in opposing phase between the grid and anode. Regeneration and increased gain are obtained in the prior art tubes by energy transfer between a pre-bunched beam and an r.f. field in the grid-anode space. To achieve this regeneration and increased gain, a driver circuit for the prior art tubes becomes electrically complex and difficult to design. Considerable time and effort for empirical design of the driver circuit and tube are necessary to achieve the desired results. It is difficult to adjust the driver cavity and tube parameters to achieve the optimum relative intensity and phase relation of the electric fields in the two r.f.-field regions. It is usually necessary to provide numerous tuning stubs and/or other variable resonant structures to provide the optimum relation. Electrons leaving the grid and accelerated toward the anode are bunched while traversing an interaction region between the grid and cathode. Any impedance presented to the electrons by either free space or resonant modes in surrounding metal or dielectric containers causes r.f. radiation and/or oscillation. This reduces the tube power gain or interferes with other equipment. Previously this problem was handled by reducing the r.f. grid-anode gap impedance substantially to zero by bypassing it with a blocking capacitor or by connecting the grid-anode gap to low impedance coaxial or strip line open-ended resonant by-pass circuits. Whatever approach is taken, full beam voltage, e.g. 32 kV or 85 kV, appears across the grid-anode gap and must be considered, as must the r.f. voltage. The blocking capacitor or by-pass circuit must be in a potting compound to minimize and preferably eliminate high voltage, D.C. arcing. There are several disadvantages in connecting the blocking capacitor or by-pass circuit between the grid and anode. Potting of the high voltage capacitors and other types of by-pass circuits capable of handling 32 or 85 kV is a problem; reliable arc-free operation is difficult to obtain. In addition, power gain is reduced because the potting compound is lossy. While tuning the grid-anode gap with open resonant lines makes voltage isolation relatively easy, such structures require extra space, tuning procedure and controls. The invention is set out in claim 1. In one preferred embodiment, the coupler includes a loop in a space between inner and outer coaxial metal signal coupling tubes having a length of about nλ/4 between the grip and loop, where λ is the wavelength of a frequency in the band, and n is an odd integer. The inner and outer tubes are respectively electrically connected to the cathode and grid. The grid and outer coaxial tube are DC isolated from the cathode and inner coaxial tube, enabling a DC bias voltage to be applied between grid and cathode and the cathode to be at a high negative DC voltage (e.g. 85 kV or 32 kV) relative to the preferably grounded anode. Preferably a DC bias connection is provided for the grid on the outer tube at a position n 1 λ/4 from the grid, where n 1 is an odd integer less than n; this position minimizes the r.f. voltage coupled to a source of the DC bias. In other embodiments, the coupler includes a grounded coaxial cable having inner and outer conductors connected to the signal source. The inner conductor is connected to a first metal face spaced from a second opposed metal face by a solid dielectric. The outer conductor is connected to a third metal face spaced from a fourth opposed metal face by the solid dielectric. The third and fourth faces respectively surround the first and second faces. The dielectric extends beyond the periphery of the metal faces so a substantial DC voltage can be established between the faces; the first and third faces are at DC ground potential while the second and fourth faces are at high negative DC voltages. The second and fourth faces are respectively at common ends of interior and exterior coaxial metal tubes forming a half-wavelength coaxial coupler. The other ends of the interior and exterior tubes are respectively connected to the cathode and grid. For wide bandwidth applications, e.g., transmitters of different UHF television stations, the coupler resonant frequency can be changed substantially. One way of varying the coupler resonant frequency is to form the coupler as a pair of variable length concentric metal tubes that are electrically insulated from each other for DC; fine tuning is provided by a capacitor plate transversely movable between the tubes. In another arrangement, a secondary cavity is electromagnetically coupled to the coupler. A shorting plunger in the secondary cavity is translated to effectively change the electrical length of the secondary cavity and the coupler resonant frequency. In the most preferred embodiment the tubes are fixed in position and have a fixed length. Metal fingers, functioning as inductive elements extending between the inner and outer tubes, are positioned at different places along the lengths of the tubes to change the coupler resonant frequency. Another aspect of the invention includes a vacuum tube of the aforementioned type wherein DC bias voltages for the grid and cathode are supplied by first and second leads connected to inner and outer r.f. signal coupling metal coaxial tubes at positions n 1 λ/4 from the grid and cathode, while heater current is supplied via a third lead that extends through the interior tube at a position n 1 λ/4 from the grid and cathode, where n 1 is an odd integer and λ is the wavelength of the signal. Such an arrangement minimizes the r.f. voltage on these leads because r.f. voltage is at a minimum at n 1 λ/4 from the grid and cathode. Examples of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a sectional view of one embodiment of a vacuum tube incorporating the present invention; Fig. 2 is a schematic view of a structure wherein the resonant frequency of a half-wavelength input coupler is varied by effectively changing the coupler length; Fig. 3 is a schematic view of a structure wherein the resonant frequency of an input coupler is effectively varied by changing the length of a quarter-wavelength secondary coupler; and Fig. 4 is a schematic view of a structure wherein the input coupler resonant frequency is varied by inductively loading a half-wavelength coupler. It is to be understood that elements designated by the same reference numerals in different figures of the drawing identify the same elements. Reference is now made to Fig. 1 of the drawing, a sectional view of a vacuum tube in accordance with one embodiment of the present invention, particularly adapted for deriving a relatively narrow bandwidth sinusoidal type wave that is applied to a particle accelerator stage. The tube of Fig. 1 includes coaxial input connector 10 which is connected to coupling loop 12, coupled to coaxial non-regenerative coupler 14, in turn connected to grid-cathode assembly 16. Electrons from the cathode of assembly 16 are density modulated by the grid of the assembly and the resulting electron bunches are accelerated by the DC field between the grid and grounded accelerating anode 18; for the particle accelerator application, the voltage between grid-cathode assembly 16 and anode 18 is on the order of 85 kV. Electrons passing through aperture 19 in anode 18 traverse an output resonator, thence are incident on a collector. The resonator includes an output loop and variable tuning capacitor including a plate that is movable transversely of the vacuum tube center line 28. A vacuum is provided in the volume subsisting between grid-cathode assembly 16 and a collector, while most of coupler 14, loop 12 and connector 10 are at atmospheric pressure or slightly above. Anode 18 and the exterior of metal housing 32 for loop 12, coupler 14 and assembly 16 are maintained at ground potential, while grid-cathode assembly 16 is maintained at approximately -85 kV. The grid of assembly 16 is maintained at a voltage of approximately -280 V relative to the cathode. DC bias for the grid of assembly 16 is applied to single "live" terminal connector 36 mounted on exterior wall 31 of housing 32, while bias voltage for the cathode of assembly 16 and energization current for the cathode heater are applied to two "live" terminal connector 38 on housing wall 31. Tuning for the vacuum tube of Fig. 1, over a relatively narrow frequency range, is provided by moving metal plate 40 transversely of centre line 28 in coupler 14. The grid of assembly 16 is fabricated of non-electron emissive material, such as pyrolytic graphite or molybdenum coated with zirconium, and spaced from the assembly cathode by a distance no greater than the distance an electron emitted from the cathode can reach the grid in a quarter cycle of the signal applied to connector 10. This type of construction is described in the aforementioned patents. The grid and cathode of assembly 16 respond to the signal coupled to them via coaxial connector 10 to current modulate the linear electron beam emitted by the cathode and accelerated by anode 18 to the collector. The resulting electron bunches propagating from the grid of assembly 16 and through opening 19 in anode 18 interact with resonant modes of the structures surrounding the region between the grid and anode 18 to cause r.f. fields at many frequencies to be established in the interaction region. The interiors of housing walls 31, in the vicinity of grid-cathode region 16 and anode 18, are covered with r.f. absorbers 42, preferably ferrite tiles. Ferrite, r.f. absorbing tiles 42 basically surround the interaction region between assembly 16 and anode 18 to absorb any potential r.f. fields generated by the bunched electrons. It has been found that the r.f. absorbing capabilities of tiles 42 are such that there is no need for assembly 16 and anode 18 to be shunted by a capacitor or coaxial or strip line open-ended resonant circuits, as was necessary in the prior art. The r.f. absorbing lossy ferrite tiles 42 heavily load the interaction region between assembly 16 and anode 18 so a resonant impedance cannot be formed in the interaction region. Because the r.f. fields in the interaction region are absorbed by ferrite tiles 42, they are not reflected back into the interaction region and are decoupled from assembly 16, anode 18 and the output cavity. The power gain of the tube including assembly 16, anode 18, the output cavity and the collector is thereby maintained at a relatively high level and interference with other equipment does not occur because r.f. fields produced in the interaction region are absorbed by tiles 42. There are certain problems in coupling the r.f. signal connected to connector 10 at basically zero DC voltage to the high negative DC voltage (for example, -85 kV) of assembly 16. According to an embodiment of the invention, this problem is resolved in the tube of Fig. 1 by a structure wherein coaxial connector 10 includes a center metal conductor and an outer, grounded conductor. A suitable coaxial cable connects an r.f. source having a relatively fixed known frequency to one end of each of the conductors. The other end of the center conductor is connected to one end of metal loop 12, having another end connected to the outer conductor. Metal loop 12 is surrounded by a polytetrafluoroethylene dielectric case that also surrounds a substantial portion of the outer conductor. Loop 12 extends parallel to center line 28 and is magnetically coupled to coupler 14, that is resonant to the frequency of the source connected to connector 10. Coupler 14 includes an outer metal tube 56 and interior tube assembly 58, a tube 56 and a tube assembly 58 both have a circular cross section and are concentric with and surround center line 28. The tube assembly 58 includes exterior metal tube 60 extending from the vicinity of loop 12 to the vicinity of assembly 16. The tube assembly 58 also includes a first relatively short metal tube 64 that is inside of and is mechanically separated from the tube 60 by a dielectric, preferably KAPTON sleeve. The sleeve enables aligned portions of both tubes 60 and 64 to be at substantially the same r.f. potential and at different DC potentials. The end of the metal tube 60 proximate loop 12 abuts a metal end cap 62 including a flange for a centering tube 60. Thereby, the end of this tube 60 and the end cap 62 are at the same r.f. voltage. The end cap 62 has an opening for receiving conduit 34 so air can be pumped through the tube 60. Conduit 34 is made of an electrical insulator so tube 60 and cap 62 can be biased to a high negative DC voltage relative to grounded housing 32. Loop 12 is positioned between metal tubes 56 and 60 so the r.f. signal supplied to connector 10 is magnetically coupled as an r.f. field by loop 12 to tubes 56 and 60. Tube 56 and the tube assembly 58, together with grid-cathode connector assembly 68, form a coaxial resonant transmission line having a length equal to an odd quarter multiple of a wavelength of the r.f. signal supplied to connector 10; preferably, the coaxial resonant structure between loop 12 and assembly 16 has a length of 3λ/4, where λ is the r.f. source center frequency. Thereby, the r.f. voltage of metal plate 62, at the end of coupler 14 remote from grid-cathode assembly 16, has a minimum value and there is a maximum r.f. voltage at the opposite end of the transmission line, where assembly 16 is located. Fine control for the frequency of coupler 14 is provided by moving capacitor plate 40 (fig. 1) transversely of center line 28 between tubes 56 and 60 during initial installation of the tube. Outer conductor 56 is connected to the arcuate grid of assembly 16 via a metal frusto-conical cup. Another arcuate cathode of assembly 16, positioned so it is generally parallel to the first grid, is connected to the first tube by a second metal tube, having an interior wall portion abutting against and bonded to a dielectric plate that forms a portion of a vacuum seal for the vacuum tube interior. A portion of the second metal tube exterior wall abuts against one edge of a dielectric washer, forming an additional portion of the vacuum tube vacuum seal. The same washer has an exterior edge bonded to the interior wall of the cup. The interior wall of the dielectric plate is bonded to a wall of a second metal cup, having a bottom face connected to one end of the heater wire, having another end connected to the interior wall of the second metal tube. The vacuum tube vacuum seal also includes a dielectric frusto-conical ceramic shell 86 extending between a metal flange 88, in turn connected to the bottom portion of metal tube 56. The other end of the ceramic shell 86 is bonded to anode 18. The heater wire includes a coiled portion in proximity to the cathode, so heat radiated from the heater wire causes electrons to be emitted from the cathode. A high DC voltage (e.g., -85 kV) supply for assembly 16, is applied via connector 36 (fig. 1) and electrically insulated lead 90 in a cable 92 to metal tube 56 at a point a quarter wavelength away from grid-cathode assembly 16. The connection of lead 90 to tube 56 at this point substantially decouples r.f. voltage at the grid from the DC source connected to connector 36. The DC voltage on lead 90 is decoupled from wall 32 and DC coupled to the grid via tube 56 and the dielectric cup. Current for the heater wire and bias voltage for the cathode (about 275 volts DC greater than the voltage of the grid) are respectively supplied via two electrically insulated leads one of which is 96 of cable 98. Cable 98 extends between connector 38 (fig. 1) and connector 100, mounted on wall 56 so the leads are DC decoupled from the wall. Insulated leads such as 96 extend along the exterior of tube 56 to flange 88, thence through an opening close to the bottom of tube 56 radially toward center line 28. Leads such as 96 are respectively connected to the cap and the second tube with the lead extending through an opening in tube 64 outside of the vacuum tube. To cool the portion of the tube vacuum envelope adjacent grid-cathode assembly 16, air is pumped via scoop 30 (fig. 1) into housing 32, having a square cross section. The air flows through apertures in tube 56 close to the assembly, thence through aligned apertures 103 in tubes 60 and 64, as well as the KAPTON sleeve, and to the interior of coupler 14 to conduit 34. The vacuum tube illustrated in Fig. 1 has been found to provide admirable results in powering a particle accelerator. The tube is easily adjusted for frequency over a narrow band (e.g. at ± 2 mHz centered on 267 mHz) suitable for particle accelerator applications. The vacuum tube has adequate power gain, without high voltage DC breakdown problems, and does not require a by-pass capacitor or other circuit elements to be connected in shunt between the grid and cathode to minimize r.f. radiation in an interaction region between grid-cathode assembly 16 and anode 18. In accordance with other embodiments of the invention, the device illustrated in Fig. 1 is modified so it can be used as a power output tube of UHF television transmitters over the entire UHF television broadcast spectrum. Such a device is advantageously easily adjusted on site, to be acceptable to UHF broadcasters. Typically, UHF transmitters have a 32 kV potential difference between the anode and grid-cathode assembly, and each tube provides approximately 60 kW of r.f. output power. These characteristics are provided by the tubes of the other embodiments. Specific structures enabling the basic structure of the input portion of an electron tube to be tuned over the UHF spectrum are illustrated in Figs. 2-4. To simplify the drawing, the structures illustrated in Figs. 2-4 do not include the output cavity and collector, i.e., the circuitry downstream of the anode. The structures illustrated in Figs. 2-4 are shorter in length and are tunable over a much broader frequency range than the device illustrated in Fig. 1, while providing the advantages of the tube of Fig. 1. In the vacuum electron tube of Fig. 2 an r.f. signal, e.g., a television signal, is coupled to coaxial line 110, including inner and outer metal conductors 112 and 114 coaxial with the electron tube center line or axis 116. The inner conductor 112 is maintained in place by a dielectric spacer insulator and is electrically connected to one end of metal plunger 120. Plunger 120 is translatable back and forth along axis 116, as indicated by arrow 122, by a suitable drive mechanism (not shown). Plunger 120, surrounded by metal cup 124, is centered on axis 116 by a dielectric washer, having inner and outer radii respectively contacting the plunger 120 outer wall and cup 124 inner wall. Cup 124 includes radially extending metal flange 128, having an outer periphery that is spaced from the side wall of the metal container. Plunger 120 includes radially extending flange 123 and planar face 125 extending at right angles to center line 116. Face 125 and a corresponding, but opposite, face of metal plate 134 provide capacitive coupling to cathode 136 for the r.f. signal connected to coaxial line 110. Cathode 136 is closely spaced to grid 138, as described supra , for the cathode and the grid. Face 125 and plate 134 are separated from each other by dielectric, preferably TEFLON, plate 140, typically having a thickness of between 30 and 60 mm and a diameter so the periphery thereof extends substantially beyond the periphery of flange 128. Dielectric plate 140 is sandwiched between opposite faces of flanges 128 and 142, which extend radially from the end of metal tube 144. Plate 140 has a geometry and is constructed such that breakdown does not occur through it even though a first flange 128 is at DC ground while a second flange 142 is at a high voltage, such as -32'kV. Tube 144 forms the exterior of a resonant coaxial half-wave coupler between face 125 and grid 138. A half wavelength coupler is employed to maximize the grid cathode r.f. voltage of the capacitive coupling from face 125 to plate 134. The coupler of Fig. 1 has a length of 3λ/4 or some other odd multiple of a quarter wavelength to maximize the grid-cathode r.f. voltage of the magnetic coupling from loop 12 to tubes 56 and 60. As illustrated in Figs. 3 and 4, the half-wave coupler 143, including tube 144, also comprises the interior tube assembly, formed by metal tube 150, integral with end plate 134 and separated from interior metal tube 152 by dielectric, preferably KAPTON, sleeve 154. Tubes 144, 150, 152 and sleeve 154 are all concentric with axis 116. Sleeve 154 provides DC isolation between tubes 150 and 152, while enabling aligned parts of these tubes to be at substantially the same r.f. potential. The end of tube 144 remote from flange 142 is DC connected by a frusto-conical cup 158 to grid 138. R.f. coupling is provided from an outer conductor 114 to grid 138 via the wall of cup 124, flange 128, through the gap between flanges 128 and 142 formed by dielectric plate 140, and along the lengths of tube 144 and the frusto-conical cup 158. R.f. coupling is provided from the inner conductor 112 to cathode 136 via plunger 120 and flange 123 thereon, to plate 134 via dielectric plate 140, thence to tube 150, across sleeve 154 to tube 152. The end of tube 152 extending beyond tube 150 is connected by a radially biased metal leaf spring assembly to a third metal tube, in turn connected to cathode 136. Electron bunches in a linear electron beam passing through grid 138 are accelerated by a grounded anode to pass through an opening in the anode into an output cavity, and thence to a collector, as described in connection with Fig. 1. The grounded anode is connected to one edge of a metal side wall having an opposite edge connected to a metal lid of the container. To establish the beam, cathode 136 is heated by the heater, having opposite ends respectively connected by two wires to a first metal cup and the third metal tube. Cathode 136, grid 138, the heater and the space between these elements to the interior face of the anode are in a vacuum formed by a seal between the third metal tube and the first metal cup by a dielectric washer and a metal radial leaf spring. The vacuum seal is also formed by two metal rings, between which another dielectric washer is wedged; these two rings have inner and outer edges bearing against the outer and inner peripheries of the third tube and the shell 158. The vacuum seal is completed by longitudinally extending the dielectric tube, having opposite ends connected to the fourth and fifth metal tubes, in turn connected to the anode and a metal flange at the end of the shell remote from grid 138. To obviate the need for a circuit element to shunt grid 138 and the anode and improve efficiency, the side walls of the container are lined with r.f. absorbing ferrite tiles, which perform the same function as the ferrite tiles in the embodiment of Fig. 1. Grid 138 is maintained at -32 kV relative to the grounded anode by connecting one end of a first electrically insulated lead of a cable to the exterior wall of tube 144, at a position removed from grid 138 by approximately one-quarter of a wavelength of the r.f. signal coupled to line 110. This cable also includes leads three and four that are insulated from each other and the lead. Leads three and four respectively supply bias voltage to cathode 136 and energizing current to the heater. Leads three and four extend through an aperture in tube 144, with the ends of leads three and four respectively connected to tube 152 and the first metal cup. The leaf spring is connected to tube 152 and lead four extends through a hole in tube 152 at positions removed from cathode 136 by about one-quarter of a wavelength of the r.f. signal coupled to the insulator. The bias voltage on lead three is supplied by tube 152 to cathode 136 by way of a metal spring finger 156 and the third metal tube 160. The current flowing in lead four is coupled to the heater via the first metal cup and lead six and from the heater to the third metal tube via lead seven. A cable and the leads therein extend through an aperture in the side wall of the housing to the terminal block, mounted on the exterior of the housing wall. The r.f. voltages on leads one and three are minimized because these leads are respectively connected to tubes 144 and 150 at positions a quarter wavelength from the grid-cathode assembly. The r.f. voltage on lead four is minimized because this lead goes through a hole in tube 152 at a position a quarter wavelength from the grid-cathode assembly and is r.f. shielded inside tube 152. The grid-cathode region of the vacuum tube illustrated in Fig. 2 is cooled in a manner similar to that illustrated in Fig. 1. To this end, a conduit extends through suitable, aligned apertures in tubes 144 and 152, to the interior of tube 152 and openings are provided in tube 152 in the vicinity of the metal spring finger and the radial leaf spring. The conduit extending through tubes 144 and 152 in the vicinity of plate 134 extends through an aperture in the housing to a pump outside of the housing. Air flowing out of the apertures in tube 152 in the vicinity of the metal spring finger and the radial leaf spring leaks to the atmosphere through openings in tube 144 and through housing 130. The structure illustrated in Fig. 2 has certain advantages over that illustrated in Fig. 1. The Fig. 2 structure is smaller, since the coaxial coupler is basically a one-half wavelength transmission line, while the coupler illustrated in Fig. 1 is a three-quarters wavelength line. In addition, relatively expensive and cumbersome loop coupler 12 of Fig. 1 is replaced by the smaller and less expensive capacitive coupling through the dielectric of TEFLON plate 140. Structure generally illustrated in Fig. 2 is particularly adapted to be set to any frequency in the UHF television band, for television broadcast purposes. Structures illustrated schematically in Figs 2-4 can be used to set the operating frequency of the resonant coupler between line 110 and cathode 136 and grid 138. In each of the embodiments of Figs. 2-4, plunger 123 and face 125 thereof are translatable relative to metal plate 134 along axis 116 by suitable means of a type known to those of ordinary skill in the art. Movement of face 125 relative to plate 134 adjusts the impedance between line 110 and the half-wavelength coupler including tubes 150, 152 and 144 to provide a proper impedance match. In the structure illustrated schematically in Fig. 2, the resonant frequency of the half-wavelength coupler between face 125 and cathode 136 and grid 138 is changed by varying the effective lengths of the metal tubes between dielectric plate 140 and the grid and cathode. To these ends, fixed length tubes 144 and 152 are respectively replaced in Fig. 2 by telescoping metal tubes 202 and 204. Tube 204 has three nested, telescoping sections (not shown) that are slidable relative to each other in the direction of axis 116, while exterior tube 202 includes two nested slidable sections (not shown). The sections of telescoping tubes 202 and 204 are coupled to each other by suitable mechanical means (not shown) so that as the length of one tube is changed, the length of the other tube varies accordingly. Adjustment of the effective lengths of tubes 202 and 204 sets the resonant frequency of the coupler between plate 140 and cathode 136 and grid 138 to the approximate resonant frequency of the signal being handled by the tube. More precisely, fine tuning is provided by moving metal plate 206 transversely of center line 116 between metal tubes 202 and 204. The structure of Fig. 2 is considerably easier to adjust than the prior art regenerative coupler. However, it is costly to provide the telescoping structures and the mechanisms for moving them. To overcome some of these problems with the device illustrated in Fig. 2, the structure of Fig. 3 was developed. In Fig. 3, plate 123 and the remaining elements "below" plate 140 are in secondary, quarter-wavelength resonant coupler 207 and the fixed length, fixed position tubes 144, 150 and 152 are employed to form a half wavelength primary resonant coupler. In addition, movable metal plate 206 is retained, as is the translatable feature of face 125 on plate 123 of plunger 120. In the tube of Fig. 3, coupler 207 includes the coaxial cylindrical metal wall of plunger 120 and outer metal tube 210. Metal, shorting disc 208 extends between the wall of metal plunger 120 and the wall of metal tube 210. The r.f. signal to be amplified is coupled to the secondary cavity by a coaxial cable having a center conductor 212 connected to the cylinder of plunger 120 and an outer conductor 213 connected to the wall of tube 210. Shorting disc 208 is set at different positions along the lengths of the cylinder of plunger 120 and tube 210 by any suitable means (not shown) to control the resonant frequency of secondary coupler 207. The position of shorting disc 208 is predetermined for each of the possible operating frequencies of a UHF television transmitter. After disc 208 has been set in position, face 125 is translated relative to dielectric plate 140. The position of metal plate 206 is then adjusted. Iterations in the positions of face 125, plate 206 and possibly shorting disc 208 are made until the desired operating parameters are attained. While the structure of Fig. 3 is mechanically simpler than the telescoping tube structure of Fig. 2 and adjustment of the tube to achieve proper operating characteristics is somewhat simpler than the structure of Fig. 2, the structure of Fig. 3 is considerably larger than that of Fig. 2 because of the inclusion of coupler 207. A structure which is mechanically simpler and easier to adjust the resonant frequency of the half wavelength coupler than the structures of Figs. 2 and 3 and is about the same size as the Fig. 2 structure is illustrated in Fig. 4. In Fig. 4, secondary resonant coupler 207 is not used; instead, the previously defined half wave resonant structure for coupling the signal to the region between face 125 and plate 134 is employed in Fig. 7. In addition, fine tuning is provided by metal plate 206, in the same manner as described in connection with Figs. 2 and 3. Approximate tuning of the half wavelength input resonant coupler for the carrier frequency of each of the UHF television channels is attained by selectively inserting one or more inductive, metal (preferably brass) tuning plugs, e.g. plugs 214 and 216, at discrete positions between fixedly mounted and fixed length inner and outer tubes, 144, 150 and 152. To these ends, tubes 144, 152 and 154 include aligned apertures (having positions shown by dotted lines 218) into which the inductive metal plugs are selectively inserted. The plugs are spring biased by a conventional structure (not shown), against the walls of tubes 144, 150 and 152 and dimensioned so they form inductive shunts between exterior tube 144 and one of the interior tubes 150 or 152; typically, the plugs are formed as cylinders having a diameter such as 2.29 mm (0.090 inches). A different carrier frequency for each UHF television broadcast carrier is associated with different combinations of the positions of the apertures along center line 116. Prior to delivery of a particular vacuum tube to a particular UHF television transmitter, one or more of the plugs are appropriately inserted and secured in the appropriate apertures. Upon delivery and connection of the tube to its load, it is merely necessary to adjust the position of face 125 relative to dielectric plate 140, to provide impedance matching to the transmitter load and to adjust the position of plate 206 for fine tuning.

Claims: A vacuum tube for handling an r.f. signal having a predetermined frequency range comprising a cathode for emitting a linear electron beam, a non-electron emissive grid for current modulating the beam, the grid being positioned from the cathode no farther than a distance in which electrons emitted from the cathode can travel in a quarter cycle of the r.f. signal, an anode (18) for accelerating the beam, an electrode for collecting the beam, an output cavity resonant to the frequency of the r.f. signal positioned between the grid and electrode for collecting the beam, and a coupler (14) responsive to the signal connected to the grid and cathode (16) so electrons from the cathode upon passing through the grid and accelerated toward the anode (18) are in bundles in an interaction region between the anode (18) and grid to cause r.f. fields that are responsive to the r.f. signal to be derived in the interaction region, and characterised by r.f. absorbing material (42) substantially surrounding the interaction region for absorbing the r.f. fields that are responsive to the r.f. signal so there is non-regenerative coupling of the r.f. signal to the region. The vacuum tube of claim 1 wherein the coupler (14) includes an input cavity resonant to the frequency of the r.f. signal. The vacuum tube of claim 1 or 2 wherein the absorbing material (42) includes ferrite tiles surrounding the interaction region. The vacuum tube of claim 1, 2 or 3 wherein the coupler (14) includes inner (58) and outer (56) coaxial metal tubes forming a resonant line, the outer and inner tubes being electrically connected to the grid and cathode (16) respectively, the grid and outer coaxial tube (56) being DC isolated from the cathode and inner coaxial tube (58) enabling different DC voltages to be applied to the grid and cathode (16). The vacuum tube of claim 4 further including a DC bias connection (36, 90, 92) for the grid disposed on the outer tube (56) at a position n 1 λ/4 from the grid, where λ is the wavelength of the r.f. signal in the predetermined frequency range, and n 1 is an odd integer. The vacuum tube of claim 4 or 5 further including means (30, 32, 34) for supplying a cooling fluid to the interior of the inner tube (58). The vacuum tube of claim 4, 5 or 6 wherein the coupler (14) includes a loop (12) in a space between the outer and inner metal tubes (56, 58) at an end of the outer and inner metal tubes remote from the grid and cathode (16), the resonant line having a length of about nλ/4 between the grid and loop, where λ is the wavelength of the r.f. signal in the predetermined frequency range, and n is an odd integer. The vacuum tube of claim 1, 2 or 3 wherein the coupler (14) is resonant to the frequency of a source of the signal, and further including means for changing the resonant frequency of the coupler (14). The vacuum tube of claim 8 wherein the coupler (14) includes a pair of concentric metal tubes (56, 58) electrically insulated from each other for DC current flow, and the changing means includes a metal plate (40) movable transversely between the tubes. The vacuum tube of claim 8 wherein the coupler (14) includes a pair of concentric metal tubes (56, 58) and the changing means changes the lengths of the tubes. The vacuum tube of claim 10 wherein the changing means further includes a metal plate (40) movable transversely between the tubes. The vacuum tube of claim 8 wherein the coupler includes a pair of fixed length, fixedly positioned concentric metal tubes (56, 58) electrically insulated from each other for DC current flow, and the changing means includes a metal, inductive structure extending between the tubes and at different locations along the lengths of the tubes. The vacuum tube of claim 12 wherein the changing means further includes a metal plate (40) movable transversely between the tubes. The vacuum tube of claim 8 further including a secondary cavity electrically coupled to the coupler, the means for changing including a shorting plunger (120) in the secondary cavity translatable relative to the secondary cavity to effectively change the electrical length of the secondary cavity. The vacuum tube of claim 14 wherein the coupler includes a pair of fixed length, fixedly positioned concentric metal tubes (56, 58) electrically insulated from each other for DC current flow. The vacuum tube of claim 1, 2 or 3 wherein the coupler includes a low voltage coaxial cable (110) having inner and outer conductors connected to a source of the r.f. signal, the inner conductor (112) being connected to a first metal face (125) spaced from a second opposed metal face (134) by a solid dielectric (140), the outer conductor (114) being connected to a third metal face (128) spaced from a fourth opposed metal face (142) by the solid dielectric (140), the third (128) and fourth (142) faces respectively surrounding the first (125) and second (134) faces, the dielectric (140) extending beyond the periphery of the metal faces so a substantial DC voltage can be established between the faces; the first (125) and third (128) faces being at DC ground potential while the second (134) and fourth (142) faces are at high negative DC voltages, the second (134) and fourth (142) faces being respectively at common ends of inner (204) and outer (144) coaxial metal tubes forming a half-wavelength coaxial coupler, the other ends of the inner and outer tubes being respectively connected to the cathode (136) and grid (138). A vacuum tube of claim 4 further including a heater for the cathode positioned in close proximity to the cathode, and wherein the resonant line formed by the inner and outer coaxial metal tubes has a length of at least λ/2 where λ is the wavelength of the r.f. signal, the coupler further includes first, second and third leads for respectively biasing the grid and cathode and for supplying current to the heater, the first and second leads being respectively connected to the outer (56) and inner (58) metal tubes at positions approximately n 1 λ/4 from the grid and cathode and the third lead extending through the inner tube at a position approximately n 1 λ/4 from the grid and cathode, wherein n 1 is an odd integer. The vacuum tube of claim 17 wherein the coupler (14) includes a loop (12) in a space between the outer and inner metal tubes at an end of the outer and inner metal tubes remote from the grid and cathode, the resonant line having a length of about nλ/4 between the grid and loop, where λ is the wavelength of a frequency in the predetermined frequency range, and n is an odd integer. The vacuum tube of claim 17 wherein the coupler (14) includes a low voltage coaxial cable (110) having inner and outer conductors connected to a source of the r.f. signal, the inner conductor (112) being connected to a first metal face (125) spaced from a second opposed metal face (134) by a solid dielectric (140), the outer conductor (114) being connected to a third metal face (128) spaced from a fourth opposed metal (142) face by the solid dielectric (140), the third (128) and fourth (142) faces respectively surrounding the first (125) and second (134) faces, the dielectric (140) extending beyond the periphery of the metal faces so a substantial DC voltage can be established between the faces; the first (125) and third (128) faces being at DC ground potential while the second (134 and fourth (142) faces are at high negative DC voltages, the second (134) and fourth (142) faces being respectively at common ends of the inner (204) and outer (144) coaxial metal tubes forming a half-wavelength coaxial coupler, the other ends of the inner and outer tubes being respectively connected to the cathode (136) and grid (138).

Inventors: SHRADER MERRALD B, SHRADER, MERRALD B.
Applicants: COMMUNICATIONS & POWER IND INC, COMMUNICATIONS & POWER INDUSTRIES, INC.
Application year: 1994
Main classification: H01J25
IPC codes: H01J23, H01J25